Scientists concerned with the biological effects of climate change are focusing on what some call “the grass of the sea.” These are tiny water plants known technically as phytoplankton. Like the green grass on which cattle feed, these little plants are at the base of many food chains in lakes and the ocean. Other tiny animals feed on them and, in turn, become food for larger critters. Knowing how phytoplankton’s abundance is changing in different locations is crucial to understanding what climate change may be doing to life on our planet.

Isolating climate’s role in plankton changes won’t be easy. As a review of the subject earlier this month in Science noted, the tiny plants are buffeted by many factors besides systematic climate change. For example, cleaning up runoff from farms can reduce the nitrogen and phosphorus that feeds plankton blooms. Fishing also can disrupt food chains down to the level where there are fewer grazers that eat the tiny plants.

Martin Montes-Hugo at Rutgers University and colleagues have found 30 years of satellite data and field studies a potent tool for cutting through that complexity. Satellites traced phytoplankton abundance by sensing the green cast of their chlorophyll. The scientists think these data reveal the hand of climate in phytoplankton changes off the western Antarctic Peninsula. They recently explained why in Science.

The data show a 12 percent decline in phytoplankton in the area over the 30-year period. The distribution of the tiny plants has also changed with declines in the northern part of the peninsula and increases to the south. The researchers also noted that the “cold-dry polar-type climate” that once characterized the region is morphing into a “warm-humid sub-Antarctic-type.”

Add it all up, Dr. Montes-Hugo says, and “we’re showing for the first time that there is an ongoing change in phytoplankton concentration and composition along the western shelf of the Antarctic Peninsula that is associated with a long-term climate modification.” His coauthor, Hugh Ducklow at the Marine Biological Laboratory in Woods Hole, Mass., draws a broader conclusion: “Now we know that climate changes are impacting at the base of the food web and forcing their effects on up through the food chain.” He adds, “Martin Montes-Hugo’s elegant work, utilizing different satellite streams of data, nailed that down.”

Montes-Hugo also speculates that “these phytoplankton changes may explain in part the observed decline of some penguin populations.” Adelie penguin populations have dropped as their familiar dry Antarctic climate has become warmer and more humid. Changes to their fish food supply due to changes in the offshore food-chain base may be partly to blame.

Dee Boersma at the University of Washington in Seattle has documented how seriously climate-induced change to food supply can affect penguins. A colony of Magellanic penguins she has been studying for 25 years in Argentina has declined over 20 percent in 22 years as its food supply of fish and squid moved farther north. This is forcing the penguins to forage farther for food with greater risk of starving. Dr. Boersma told a meeting of the American Association for the Advancement of Science in Chicago last February that climate was a major factor involved.